Previous work on this project has demonstrated that there are significant within-species variations in visual capacity among some species of South American monkeys, most notably the squirrel monkey (Saimiri sciureus). These variations include both visual sensitivity and color vision. The types of color vision found in these animals are indiscriminable from these characteristic of certain humans with color-defective vision. The projects planned have the following aims: (1) To expand the measurements of color vision in squirrel monkeys of Peruvian origin by behaviorally testing additional animals so as to assess the extent and nature of color-vision variation in this species and to determine if the color vision of squirrel monkeys of non-Peruvian origins are similar or different from those of Peruvian origin, i.e., to see if genetic differences are paralleled by color vision differences. (2) To investigate the biological mechanisms responsible for color vision by (a) measuring by microspectrophotometry (MSP) the cone pigments present in the retinas of animals whose color vision has been previouslY assessed and so determine for the first time the exact relationships between defective color vision and retinal photopigments, (b) by recording the activities of single nerve cells in the visual systems of squirrel monkeys to determine how neural coding for color varies among subjects having different types of color vision. (3) To try to ascertain by behavioral experiments what significance these large intraspecies variations in color vision hold for normal visually guided behavior.